[time-nuts] DMTD Mixer Terminations

Brian Kirby kilodelta4foxmike at gmail.com
Sun Feb 28 06:01:27 UTC 2010


The values in the schematics are wrong for the op amp gain.  The drawing 
was from an earlier drawing where I made a preamp to start checks on the 
mixers, and I sent it to you (Bruce G).  Thats when you determined I did 
not have enough gain to get near the noise floor.  The THAT1512/1646 ICs 
were ordered to make a new preamp for the future measurements on the mixers.

When I use the scope and check the outputs of the IC, I have 20 volts 
peak to peak, sine-wave.  I know from previous readings I see about 500 
mv p-p out of the mixer.

I went down to the bench and the resistors I used were still there (I 
bought several taped reels of Dale RN55D resistors when a local business 
went out).  I used 294 ohms and 14.9  kilo-ohms, for a gain of 50 (the 
power rails are +/- 15 volts).  Also not shown on the schematic is a 
0.47 uF cap around the 14.9 kilo-ohm resistor.  I think I was trying to 
limit the bandwidth to around 15 hertz.

Also the resistor going between the op amp and the limiting diodes was 
marked 10K, its 20K.  The diodes are 1N4148.  Corrected drawing attached.

This is what happens to time nuts who can only play on the weekend and 
stay up all night....and my employer just thinks I party too 
hard.....for Monday mornings.



Brian KD4FM



Bruce Griffiths wrote:
> The LT1037 is shown with a gain of ~1690x, if this amplifier is used 
> to amplify the beat frequency signal, it will saturate.
> Opamp recovery from saturation is poorly documented and may be very slow.
> It would be better to use some diodes in the amplifier feedback 
> network to limit the large signal gain to 5x (so that the LT1037 
> remains stable as it isn't unity gain stable).
> This will ensure a somewhat faster recovery from overload as the 
> LT1037 then avoids saturation and the opamp input stage remains in the 
> linear region.
>
> Bruce
>
> Bob Camp wrote:
>> Hi
>>
>> Assuming that the junction of the back to back diodes goes trough a 
>> chunk of coax to get to the counter:
>>
>> You are forming a low pass filter with the 10K resistor and the coax 
>> capacitance. The LT1037 is quite happy driving a 600 ohm load. You 
>> could easily drop the impedance at that point below 300 ohms. That 
>> should give you a faster edge into the counter.
>>
>> You also should check the slew rate performance of the 1037. You 
>> don't want the op amp to be slew rate limited.
>>
>> Bob
>>
>>
>> On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
>>
>>   
>>> I am in the process of designing a DMTD system.  As an experiment to 
>>> do basic measurements on the chosen mixer, I used a capacitor (0.01 
>>> uF) in series to ground with a 47 ohm metal film resistor.  Where 
>>> the capacitor and resistor meets, another resistor is attached (390 
>>> ohms) that goes to ground.  The idea is to provide a 50 ohm 
>>> termination at 20 Mhz and a lighter termination at audio 
>>> frequencies.  I seen this is a NBS note and I can say, its a 
>>> starting point for my experiments.
>>>
>>> This (my) system is designed for 10 Mhz, using a 10 hertz beat.  A 
>>> schematic is attached of what I am experimenting with at the 
>>> moment.  A HP5370B is the recording instrument.  The noise floor 
>>> from 1 days observations show  2x10-11 at 0.1 seconds, 2x10-12 at 1 
>>> sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 
>>> 7x10-16 at 10,000 secs.   It will be interesting when the project is 
>>> completed to see how much improvement there will be.
>>>
>>> As I understand (or learning..) mixer performance is the key to the 
>>> DMTD system.   It occurs to me that maybe a capacitor designed for 
>>> 50 ohms at 20 mhz may be a better termination (for the IF port) for 
>>> this mixer.  A 16 pF capacitor is 50 ohms at 20 mhz, and for 
>>> comparison at 10 hertz, it would be 100 meg-ohms, which would give 
>>> maximum amplitude at 10 hertz.   As I understand, a capacitor 
>>> terminated mixer will give a triangle wave output, which is very 
>>> beneficial to the design - as the end result is to get maximum slope 
>>> out of the mixer.  I would say, unqualified as I am, the capacitor 
>>> termination matches the 20 mhz signal, and helps attenuates the 
>>> harmonics of the mixer, and has no , or very little effect on the 
>>> audio frequencies that we are interested in.
>>>
>>> And saying/rambling on... that if maximum slope is needed, its 
>>> needed on the 10 hertz beat signal - so maybe a capacitive 
>>> termination on the 10 hertz signal only and something resistive on 
>>> the 20 mhz signal........another idea use the 16 pF direct off the 
>>> mixer, then a series resistor for isolation and then a large 
>>> capacitor on the 10 hertz beat for maximum slope.
>>>
>>> At the present, I am awaiting parts to build a low noise preamp base 
>>> on the THAT1512 so I can make better measurements on the mixer.  
>>> Bruce has provided a lot of good suggestions and helpful comments on 
>>> my project and Ulrich has provided me quite a bit of user support on 
>>> his program, Plotter.  Thanks to all.
>>>
>>> Comments ?     Brian KD4FM
>>> <DMTD_Plans.pdf>_______________________________________________
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>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>> and follow the instructions there.
>>>      
>>
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>
>
>
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